CP-24361: abstract qemu interface: rebase patch CP-24090 d1cfa14 into qemu backends

Rebase the following patch into the qemu backends:

> commit d1cfa14
> Refs: 0.9.30.1-797-gd1cfa14
> Author:     Liang Dai <liang.dai1@citrix.com>
> AuthorDate: Tue Sep 12 23:03:39 2017 +0800
> Commit:     Liang Dai <liang.dai1@citrix.com>
> CommitDate: Wed Sep 20 10:18:24 2017 +0800
>
>     CP-24090: Respond to RTC_CHANGE QMP message
>
>     For QEMU-upstream, when receives RTC_CHANGE event, write the value to
>     xenstore path /vm/<uuid>/rtc/timeoffset, which will wakeup xenstore
>     watcher in xenops_server_xen to uptdate timeoffset.
>
>     Signed-off-by: Liang Dai <liang.dai1@citrix.com>

Signed-off-by: Marcus Granado <marcus.granado@citrix.com>

Author: mg12ctx

xc/device.ml

@@ -2063,7 +2063,16 @@ module Backend = struct
 
           let qmp_event_handle domid qmp_event =
             (* This function will be extended to handle qmp events *)
-            debug "Got QMP event, domain-%d: %s" domid qmp_event.event
+            debug "Got QMP event, domain-%d: %s" domid qmp_event.event;
+            qmp_event.data >>= function
+            | RTC_CHANGE timeoffset ->
+              with_xs (fun xs ->
+                let timeoffset_key = sprintf "/vm/%s/rtc/timeoffset" (Uuidm.to_string (Xenops_helpers.uuid_of_domid ~xs domid)) in
+                try
+                  let rtc = xs.Xs.read timeoffset_key in
+                  xs.Xs.write timeoffset_key Int64.(add timeoffset (of_string rtc) |> to_string)
+                with e -> error "Failed to process RTC_CHANGE for domain %d: %s" domid (Printexc.to_string e)
+              )
 
           let qmp_event_thread () =
             while true do